1. Field of the Invention
The present invention relates to an antireflection multilayer film formed on a surface of an optical element.
2. Description Related to the Prior Art
In optical instruments, e.g., a digital camera, an image scanner, a liquid crystal display and a projector, optical elements such as lenses and various optical filters are used. Although a shape and an optical effect of the optical element varies depending on uses, in either case, it is common that a single-layer or multilayer antireflection film is applied on the surface of the optical element. This is to prevent deterioration of light utilization efficiency caused by surface reflection on the optical element.
As the antireflection multilayer film, for example, films formed by laminating plural dielectric materials whose indexes of refraction vary (so-called dielectric multilayer film) are known by Japanese Patent Laid-Open Publication No. 2002-156507 and Japanese Patent Laid-Open Publication No. 2006-119525. For the antireflection multilayer film using the dielectric multilayer film, a combination of dielectric materials and these indexes of refraction, a number of layers and a laminating order of each dielectric layers and so on vary depending on a wavelength range to be used and so on. For example, in case two kinds of dielectric materials are used, a dielectric layer including a high refractive index material and a dielectric layer including a low index of refractive material are alternately laminated.
In addition, Japanese Patent Laid-Open Publication No. 07-027902 and Japanese Patent Laid-Open Publication No. 63-075701 suggest to form one oblique deposition layer by depositing an inorganic material from a diagonal direction, as a single-layer antireflection film.
In many optical instruments, dust and dirt is easy to adhere to an optical element which is exposed. When dust and dirt adheres, as a matter of course, optical performance turns worse. In addition, the optical element itself may attract neighboring dust and dirt by electrostatic charge. Furthermore, even in case that the optical element is almost sealed up during use, the optical element will be exposed to outside air for maintenance, and dust and dirt may adhere to it.
In case dust and dirt has adhered on a surface of the optical element, dust and dirt should be removed by an air duster or the like to prevent deterioration of optical performance. However, it is not easy to completely remove dust and dirt which has adhered once. For example, minute dust and dirt often remains even after blowing air using an air duster. Therefore, it is desirable to provide a dust-resistance coating on the surface of the optical element, in addition to the above-mentioned antireflection film.
However, applying the dust-resistance coating additionally on the antireflection film will increase manufacturing cost due to an increase of manufacturing steps. In addition, the antireflection effect of the antireflection film may decrease by applying the dust-resistance coating. Furthermore, the thickness of the optical element increases by the dust-resistance coating. Accordingly, in case the dust-resistance coating is provided for all the optical elements to be used for the optical instrument, since the thickness as a whole becomes a degree that cannot be ignored, downsizing or thinning of the optical instrument may be disturbed.